Method of making honeycomb core material



Sept. 2, 1952 H. c. ENGEL ET AL 2,609,315

MACHINE FOR MAKING HONEYCOMB CORE MATERIAL Original Filed Oct. 50, 1946 4 Sheets-Sheet l INVENTORS ATTO 'EY 4 Sheets-Sheet 2 H. C. ENGEL ET AL MACHINE FOR MAKING HONEYCOMB CORE MATERIAL Sept. 2, 1952 Original Filed Oct. 50, 1946 Sept 2, 1952 H. c. ENGEL ET AL MACHINE FOR MAKING HONEYCOMB CORE! MATERIAL 4 Sheets-Sheet 5 Original Filed Oct. 30, 1946 INVENTOR A/fi/PPY C-ENGEL MLMFa 14/. 770x541. TWEOOOPE 2/ 8/17,

AORNY Sept. 2, 1952 H. c. ENGEL ET AL 2,609,315

MACHINE FOR MAKING HONEYCOMB CORE MATERIAL Original Filed Oct. 30, 1946 4 Sheets-Sheet 4 Patented Sept. 2, 1952 NT OFFICE METHOD OF MAKING HONEYCOMB CQRE MATERIAL Harry G. Engel, Towson, and Willard W. Troxell and Theodore P. Pajak; Baltimore, Md, assignors to The Glenn L. Martin Company, Middle River, Md., a corporation of Maryland Original application October 30, 1946, Serial No. 706,672; Divided and this application January 31, 1951; Serial No. 208,699

Claims. (oi.

Our invention relates to an improved method of forming honeycomb corevmaterial of resin impregnated strips bonded together to form an assembly of cells having predetermined density and flexural stiffness characteristics. 7

This is a division of application Serial No. 706,672 filed October 30, 1946, now abandoned.

Sandwich structural material wherein stid dense surface sheets are separated and stabilized by a relative thick light-weight soft core material have proved to have considerable merit when used as sandwich panels in the construction of aircraft and similar applications where weightis an important factor. However, prior to this time such core material has been limited in use due to its lack of fiexural stiffness and high cost of production.

The present invention'provides a method of forming such core material in which relatively dense high-strength fabric strips are arranged to form hexagonal shaped cells in which the full strength of the Walls of said cells are utilized to produce tensile and compressive strength in the direction of the cell axis by the provision of double and single ply wall formations in a structure characterized by having an extremely low specific gravity.

It is among the objects of our invention to produce an inexpensive light-weight core material having certain predetermined density and strength characteristics wherein strips of resin impregnated fabric are shaped and'bonded together by application of heat and pressure.

Another object of our invention is to produce a honeycomb core material characterized by certain predetermined specific gravity, flexural stiffness, and shear strength characteristics.

Another object of this invention is'to provide a method for inexpensively producing core material in which resin impregnated material is shaped and formed as strips suited for bonding together at predetermined points to form a cellular structure.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawings in which like numbers referto like parts in different views of the drawings.

Figure l is an enlarged fragmentary sectional view of the honeycomb core structure of this inlie- 33.05)

vention showing arrangement of the walls of each cell.

Figure 2 is an enlarged fragmentary sectional view of the strip material prior to shaping.

Figure 3 is an enlarged fragmentary sectional view of the same strip shaped ready for assembly.

Figure 4 is a fragmentary perspective of the core material showing its preferred embodiment as a stabilizer between two metal surface sheets.

Figure 5 is a fragmentary perspective of the core having a modified form or cell structure.

Figure 6 is a top plan view of the strip shaping machine. v

Figure 7 is a sectionalview of the machine on line 1''l of Figure 6.

Figure 8 is an enlarged fragmentary sectional view of the shaping ribs and-bars showing their meshed positions.

Referring now to Figure l, we have illustrated a fragment of the core structure if] which shows the arrangement of the cell walls attained in the finished article. Core structure Ill is'comprised of a plurality of hexagonal shaped cells H having a pair of opposed Walls of double thickness and two pairs of opposed walls of single thickness formed of suitable flexible material rendered fcrmable by being impregnated with a suitable thermosetting resin. Most satisfactory results have been obtained by the use of fabric sheets impregnated with a phenol-formaldehyde to a resin content of about by weight which has been partially cured prior to being out to the desired width for shaping. lhe fabric in its pre-. shaped form is illustrated in Figure 2 as aflat sheet i2. This fiat sheet I! is cut into the form of a continuous strip l6 suited for handling as a roll and adapted for shaping in the continuous shaping machine hereinafter described as part of this invention toprovide the shaped strip it as shown in Figure 3, having bond areas l3 longitudinahy spaced apar'tand positioned above and below the center line of the original sheet. The intervening areas between the bond areas it are shaped to form the sloped wall areas It as best illustrated in Figure 3. The configuration of the alternate bond areas !3 and Wall areas It may also be referred to as formations representing one-half of the hexagonal shaped cell H of the finished core assembly.

The plurality of hexagonal shaped cells H is attained when the shaped strip is cut to predetermined lengths at uniform severance points relative to the configuration of the one-half hexagonal figures shaped therein and stacked with their bond areas placed against each other so as to form a plurality of closed hexagonal cells shaped to the geometrical form of cell H of the core structure (Figure 1). A novel feature of this form of structure is that it i adapted for stacking with the bond areas of a plurality of succeeding strips together to provide an assembly of cells to any desired width of core material. Furthermore, by arrangement of the bond areas l3 adjacent each other a cell characterized by having a pair of opposed walls of double thickness and two pairs of opposed walls of single thickness is formed. This arrangement of the cell walls, as best illustrated in Figure 1, allows for a full utilization of the full strength of each wall with a minimum increase in the specific gravity of the assembled structure. Thi novel formation of the cell walls permits the formation of an efficient core structure having a specific gravity of less than .10. We have found that in the formation of a core material for use in separating and stabilizing aluminum and magnesium alloy sheets of conventional thicknesses, namely, .016 to .051, that cell walls formed of cotton fabric weighing substantially 12 grams per square foot impregnated with phenolic laminating varnish to a resin content of approximately 50% by weight shaped and assembled to form hexagonal cells having diameters of substantially .43 inch provides optimum fiexural stiffness and specific gravity characteristics for the finished sandwich structure. trated in Figure 4 in its preferred embodiment as a core material separating two metal sheets 90 and 9| bonded to the core ID to form an exceptionally stiff sandwich panel low in specific gravity.

In forming the core structure of this invention, we cut the flat sheet [2 to a suitable width for handling as a rolled continuous strip l5 (Figures 7 and 8) adapted for feeding into the continuous shaping machine in which the core material is formed.

A machine suitable for the practice of this invention is shown by Figures 6 and '7 mounted on a heavy rigid frame structure of conventional design having a bed plate I! extending between upright sidewall members 18 had 19 raised above the floor level and supportedby suitable channeled legs 20 arranged in pairs. Cross members 2| positioned between and welded to the legs add additional rigidity to the structure.

Side wall members l8 and H! are extended upwardly to receive a removable roller 42 adapted to position the roll 4| of strip material 16 for aligned feeding of the strip into the shaping mechanism. The strip shaping mechanism of the machine is comprised of a large cylinder 22 mounted transversely to the bed plate I! with bearings 23 and 24 carried by the side wall members l8 and I9. Around the outer periphery of cylinder 22 a plurality of shaping ribs 25 are provided in the form of gear teeth. Each rib 25 is formed with a fiat top 12 (Figure 8) and sloping sides 13 separated by a flat bottom land I4 equal in width to that of each top, it being intended that the geometry of each rib be that of one-half of a hexagonal.

The flat top surfaces 72 of the ribs 25 are shaped and dimensioned so as to produce the fiat areas 26 in one side of the shaped strip l5 (Figure 3) spaced between the bond areas i3. likewise the The structure so produced is illussloping side areas 13 correspond to the interior of the sloped wall areas 14 of the shaped strip. The bottom land areas 14 between the ribs correspond to the bond areas l3 as formed on one side of the shaped strip 15.

A driving roller 28 is spaced from the large cylinder 22 a sufficient distance to allow for meshed engagement of the ribs 25 with bars 35 corresponding in shape and size with ribs 25 and carried by a flexible endless conveyer 31. The driving roller 28 is secured to drive shaft 29 journaled in suitable bearings carried by the side walls with one end of the shaft extended beyond the side wall I8 to allow for drive attachment of pulley 30. Pulley 30 is adapted for belt, or like drive, through a second pulley 3| of a speed reduction drive mechanism 32 powered by an electric motor 33 supported by a bracket 34 bolted to side wall I8. Speed reduction unit 32 is preferably of a variable speed type, however, any conventional unit of the proper power output capable of being operated at a certain predetermined speed will provesatisfactory. Cross bars 35 corresponding in length and shape to ribs 25 are spaced and mounted on a non-stretchable flexible fabric endless conveyer belt 31 so as to provide for loose meshed engagement with the shaping ribs 25 carried'by the large cylinder 22. Each one ofthe cross bars 35 is amxed to belt conveyer 31 by screw bolts 95, or like means, inserted through the belt material. The conveyer belt 3'! is in turn supported and held in position by a stretchable endless supporting belt 33 formed of rubber or like material adapted for encirclement of the driving roller 28 and positioning roller 33 positioned under the large cylinder 22. The supporting belt is maintained at the proper operating tension by an adjustable idler 39 mounted back of the positioning roller to hold the cross bars 35 of the conveyer belt 3'! in meshed position with shaping ribs 25 for a predetermined distance around the outer periphery of the large cylinder 22. In this manner a shaping means is provided capable of maintaining a firm but resilient engagement between the shaping ribs of the large cylinder .and the bars carried by the endless conveyer belt respectively. The shaping ribs 25 and bars 35 being maintained in fixed position relative to each other provide the sloped sides and flat top and bottom shaping areas in uniform engagement for a predetermined distance. The distance determined being suflicient to allow for the strip iii of thermosetting resin impregnated fabric rendered formable by application of heat to be shaped to the contour of the shaping members and to be held to such shape until cooled sufficiently to become fixed as a continuous operation. This arrangement of a resilient supporting belt positioned to back up the shaping cross bars 35 and thereby hold them in constant meshed relationship with the ribs 25 for the required shaping distance prevents any tendency that the endless conveyer belt 31 might otherwise have to sag away from the large cylinder 22.

For convenience of feeding, the resin impregnated strip I6 is wound into a roll ll and placed above the shaping mechanism on roller 42 mounted between the side walls from whence the material can be readily unrolled as needed. The strip 15 is then carried through a heating area represented by heating unit 45 (Figures 6 and 7) which may consist of an electric heating coil, a bank of lamps, or like heating means of sufficient capacity to heat the strip to from 350 F. to 375 F. at which temperature it is rendered formable and suited for feeding between shaping ribs 25 and cross bars 35 which mesh sumciently to draw the strip in its heated condition therebetween and hold it to the desired shape while advancing the strip to the point where the meshed members separate, moving the strip material, which in its shaped condition is illustrated and referred to as shaped strip l5 (Figure 7), outwardly away from the meshed shaping members onto the platform of the machine. The shaped strip I5 is then in proper condition iorfurther fabrication except for possible slight irregularities and a glaze which customarily forms on the surface of the strip due to the curing characteristics of the thermosetting resin and in those instances where an extremely close bond is desired we have found it desirable to remove said irregularities and glaze by means of a suitably mounted sanding mecha nism. We are aware, however, that certain improved high strength adhesives may eliminate the need for the sanding operation. The sanding mechanism consists of a pair of endless sanding belts mounted for crosswise operation in contact with the top and bottom bond areas iii of the shaped strip i5 as it is moved along the machine platform 64. By operation of the sanding belts crosswise of the strip i5 theflat bond areas 13 are worked parallel to their sloped walls whereby the inherent stiffness of the walls is utilized to secure evenly sanded surfaces. The top sanding belt 46 (Figures 6 and 7) is positioned to move crosswise of strip as an endless belt in a given direction around a pair of top rollers 38 and 49 provided with suitable bearings carried by the machine side walls. The bottom sanding belt 47 is similarly mounted for movement in the oppo site direction to engage the bottomside of the strip on a pair of bottom rollers "it. Power for the operation of the sander is provided by an electric motor 58 suitably mounted to provide for direct drive through a set of pinion ears 15. The bottom sanding belt t? is driven in the pposite direction by the same motor by means of a pulley mounted on the end of the shaft of top roller ift and a like pulley mounted on the bottom roller i6 connected by a belt iii. The sander is enclosed in a suitable housing 52 and provided with an exhaust fan '79 driven by an electric motor 85 for conduction of the resin dust produced by the sander away from the machine through outlet 8!.

In order to provide for bonding of the strips together, it is necessary that a second coating of thermosetting resin be applied to the bond areas i3 and this is accomplished by introducing the shaped strip to a suitable wetting mechanism which we have illustrated as a double acting spreader having a top spreader roll 53 and bottom spreader roll 54 mounted for contact with the moving strip to apply a solution of bond material to the areas prepared for bonding on both sides of the strip as it passes therebetween. A reservoir for the bonding material solution is provided by a supply tank 55 mounted on the side wall I8 of the machine. Conveyance of the resin from the supply tank to the top roll 53 is provided for by a duct 55 equipped with a shutoff valve 57. A supply of resin to the bottom roll 54 is provided for by a dip tray 58 arranged for partial submersion of the spreader roll in the fluid supplied thereto. Resin is conveyed from the supply tank 55 to the dip tray 58 by a second duct Bl! provided with a control valve 6!. The level of resin in the tray may be automatically maintained at the right point by means of a float system (not shown) if desired". Dip tray 58 is supported under the-bottom-spreader 54 by a suitable support bracketed tothe side walls such as cross member '59. Excess resin is scraped from the top spreader roll by means of a scraper 62. No scraper is needed for the bottom roll as when operated under normal conditions there is no tendency of the strip to pickup excess resin.

Rotation of the spreader rolls 53-and 54 is ac this manner the material canbe cut to any desired lengths, however, in producing honeycomb construction material In for commercial panels, a time setting suited to cut the strip to either i, 6 or 8 foot lengths would be utilized.

The cut lengths of the shaped strips I5 are in proper position as they move from the knife to be dropped into an assembly rack or box conventionally placed at the end of the machine platform 64 supported by a' table 88. The assembly rack is provided with a floor 5?, sides 68 and 69, fixed end 10, and an adjustable end H. The adjustable end H is suitably positioned by means of adjusting screws 86 mounted in an end brace 81 to provide for proper placement; either by hand or by an automatic handling device (not shown), of the strips in stacked relationship so that the flat surfaces that provide the'bond areas 13 of each strip willfall on the bond areas of each precedin strip to provide a series of hexagonal cells therebetween. After a suitable number of strips have been assembled a top plate (not shown) adapted to hold the strips in assembled position with the bond areas [3 in contact with each other is placed on top of the assemblage 89. The assembled strips are then bonded together at the bond areas by the application of suihcient heat to completely cure the second coating of thermosetting resin applied to the strips by the spreader.

The assemblage 89, after cooling, represents the bonded cellular structure referred to as core It and is then in condition for cutting or shaping to any specific thickness required. In the economical production of core, we have found it advisable to form material to substantially the full width capacity of the shaping machine. Such an assemblage is well suited for cutting into sheets of any desired thicknesses on regular wood working equipment Furthermore, it can be stored for further working and sizing as the need arises.

Operation of the machine in the construction of cellular core material is as follows:

A continuous strip l6 of flexible thermosetting resin impregnated material cut to suitable width for passing through the machine is rolled and placed on shaft 42. The free end of the strip it is passed under the heating unit. 45 energized from a suitable source ofelectrical supply current and placed in contact, with cylinder 22] The main driving motor 33 is energized from a supply line (not shown) which in turn rotates the driving roller 28 moving shaping cross bars 35 affixed to conveyer belt 3! into meshed engagement with shaping ribs 25 positioned around the outer periphery of cylinder 22. The free end of strip I6 is thereby drawn between the shaping members and so shaped to their configuration, having. been rendered formable by the heat supplied by heating unit 45. The function of endless belt 36 is not only to act as a drivin medium upon which conveyer belt 3'! rides but also a support and holding means to hold the shaping members in position long enough for the resin carried by the strip material to cool and fix the shape of the strip to that of the shaping members. Having been so shaped, the strip material emerges as shaped strip l5 formed with a series of flat areas equally displaced outwardly from the center line of the original strip. Any irregularities as well as the glazed resin surface formed by the resin are removed from these fiat areas by the sanding belts 46 and 41 mounted and driven so as to move in opposite directions across said flat areas herein referred to and shown as bond areas l3. The shaped strip is then passed between resin spreading rolls 53 and 54 which place a fresh coating of thermosetting resin on said bond areas 13 preparing them for bonding together. The strip is then cut into suitable lengths by a cutting knife 65 arranged to operate at spaced intervals. The cut sections of strip [5 are then stacked either by hand or mechanically in a receiving form with the bond areas l3 together to form an assemblage 89 of stacked strips after which the assemblage is heated to substantially 300 F. for about 15 to minutes to completely cure and bond said strips together to form the stiff cellular core l0.

Figure 4 shows the finished core III in its preferred embodiment as a separator between metal face sheets 90 and 9| bonded thereto transversely to the axes of each cell. This form of structure allows for full utilization of the novel arrangement of the cell walls placed and arranged so as to withstand the crushing and the shear loadings common to such panels while retaining the specific gravity characteristics attainable in such a closed cell assemblage.

In constructing the novel core construction shown in Figure 5, shaped strips I5 are spaced apart by flat strips 92 of the same resin impregnated material with the bond areas l3 aligned transversely of the core. The geometry of the cells thereby produced is that of aligned one-half hexagonals separated by straight-walled partitions. This form of structure is ready of assembly and allows for full utilization of the strength in the complementary double and single walls formed therein to produce a core readily formable to double curvatures. The formation of relatively thick materials to double curvatures has always presented a serious problem in view of the well known tendency of a flat sheet to form a saddle-back anticlastic surface when subjected to couples along only two opposite edges. The physical reason is that all materials when stretched in one direction contract in a transverse direction and extend laterally when compressed. This problem is solved by the geometry of the cellular construction of Figure 5 in that the formation of the double walled areas 93 are spaced between single wall areas 94 which allows for ready distortion of the cell pattern by bending of the single wall areas between the bond areas without afiecting the load carrying ability of the structure. Therefore, the core construction is adapted for use either between fiat sheets as shown in Figure 5 or curved sheets between which the core can be readily curved to single or compound curvatures by applying compression on the side intended to fit the sheet surface having the lesser radius.

Our invention is capable of many modifications within the scope of this specification and is therefore not intended to be limited to the precise arrangement shown nor otherwise than as set out in the following claims.

We claim as our invention:

1. A method of forming structural material comprising first softening the resin in a partially cured strip of phenolic impregnated fabric by the application of heat, then forming the heated strip by passing said strip between forms shaped as half-hexagonals to provide spaced bond areas crosswise of the strip, fixing the shape of the strip by cooling, applying a coat of adhesive material to said bond areas, cutting the strip into a plurality of strips, stacking the strips with the coated bond areas in contact and bonding by the application of heat to form a plurality of hexagonal cells.

2. A method of forming cellular structural core material comprising first heating a partially cured strip of phenolic resin impregnated fabric to approximately 375 F., then shaping the heated strip into a series of half-hexagonals connected to each other by bond areas corresponding in length to one side of the hexagon, holding said strip in its shaped condition during cooling to fix its shape, applying a coat of resin to the bond areas, cutting the strip into lengths and stacking the strips with the coated surfaces of the bond areas in contact and bonding the strips to each other by the application of heat to form a plurality of hexagonal cells.

3. A method of forming cellular core material comprising first heating partially cured strips of phenolic resin impregnated fabric, then corrugating said heated strips by the application of pressure in forms shaped as half-hexagonals, preparing the crests of said corrugations for bonding by sanding and coating with bonding material, stacking the corrugated strips between alternate fiat strips and bonding the adjacent strips together at the bond areas by the application of heat to form a unitary structure having cells separated by walls having double and single thickness areas.

4. A method of forming lightweight cellular honeycomb structures from partially cured strips of thermosetting resin impregnated fabric material comprising, first heating said strips to render them formable, then forming said heated strips by passing them between forms shaped as half hexagonals to provide spaced bond areas crosswise thereof, allowing said formed strips to cool sufficiently so as to fix their shape, sanding said bond areas to remove the glaze produced on the surface of the resin, applying a coat of adhesive to said sanded bond areas, stacking said strips so as to form open-ended cells with said bond areas contacting adjacent strips and curing said adhesive to bond the adjacent strips together as a unitary structure.

5. A method of forming lightweight cellular honeycomb structures from partially cured strips of resin impregnated fabric material comprising, first softening the resin in said strips by the application of heat, then forming the heated strips by passing said strips between forms shaped as half-hexagonals to provide spaced bond areas crosswise of the strips, allowing said formed strips to cool sufiiciently so as to hold their shape, sanding said bond areas to remove the glaze produced on the surface of the resin, applying a coat of the resin impregnate to said sanded bond areas,

- stacking said strips with the coated bond areas in contact and curing said resin to bond the adjacent strips together so as to form a unitary structure having a plurality of generally hexagonal shaped, open-ended cells.

HARRY C. ENGEL. WILLARD W. 'IROXELL. THEODORE P. PAJAK.

REFERENCES CITED The following references are of record in the file of this patent:

Number 10 Number 

1. A METHOD OF FORMING STRUCTURAL MATERIAL COMPRISING FIRST SOFTENING THE RESIN IN A PARTIALLY CURED STRIP OF PHENOLIC IMPREGNATED FABRIC BY THE APPLICATION OF HEAT, THEN FORMING THE HEATED STRIP BY PASSING SAID STRIP BETWEEN FORMS SHAPED AS HALF-HEXAGONALS TO PROVIDE SPACED BOND AREAS CROSSWISE OF THE STRIP, FIXING THE SHAPE OF THE STRIP BY COOLING, APPLYING A COAT OF ADHESIVE MATERIAL TO SAID BOND AREAS, CUTTING THE STRIP INTO 